It is a general practice to use involute spur gears in a gear pump configured to move a fluid from a suction side toward a discharge side by rotation of the gears meshing with each other. This is because an involute tooth can be cut easily and allows measurement of its finished dimensions to be easily carried out, thereby making it possible to provide for a high precision gear.
On the other hand, the involute spur gears entail an adverse effect called “fluid confinement phenomenon”. During rotation of the involute spur gears, there is a period during which two pairs of teeth mesh with each other to confine the fluid therebetween. The volume of the confinement region varies with rotation of the gears, thus bringing about an inconvenience such that when the confinement region is compressed, a rise in the pressure of the fluid confined and wasteful consumption of power occur, while when the confinement region is expanded, a vacuum or air bubbles are produced.
The adverse effect of the confinement phenomenon becomes more serious as the viscosity of the fluid being delivered, or the suction or discharge pressure of the pump increases. Therefore, it is difficult to employ the involute spur gears in a pump for use in delivering high-pressure and high-viscosity fluids, such as molten resin, with pressure.
The above-described confinement phenomenon can be avoided by employing helical gears with their helix angle adjusted appropriately. In addition, a helical gear pump fails to cause the pressure of the fluid being delivered to change steeply, offers relatively smooth gear engagement, and is capable of suppressing noise and vibration.
However, since the helical gears are subjected to the action of axial thrust (thrust force) during rotation, the side faces of the respective gears are strongly pressed against each other in the axial direction to cause friction, which sometimes results in seizure. For this reason, a double helical gear is usually used which is capable of canceling each other's axial thrust (see patent document 1 for example).
Such a double helical gear is not easy to form. As is often the case in actually manufacturing the double helical gear, two helical gears which are symmetric with each other are joined together to form one double helical gear. With such a manufacturing method, the gears and gear shafts cannot but be separate members. Accordingly, machining for forming keys, key ways and the like is needed to join the gears and the gear shafts with each other. What is more, the gears and the gear shafts have increased diametrical dimensions, which will lead to an increase in the size of the pump.
The present invention, which has been made in view of the foregoing, intends to realize a gear pump which is suitable for delivering high-pressure and high-viscosity fluids, without using a double helical gear.
Patent document 1: Japanese Patent Laid-Open Publication No. HEI 08-014165